Hydrophilic polymer coated microporous membranes capable of use as a battery separator

ABSTRACT

The present invention is directed to microporous membranes having a surfactant impregnated therein which is coated on at least one surface thereof with a polymer coating, such as cellulose acetate. The polymer coating possesses functional groups in the presence of an aqueous alkaline environment which permits it to undergo hydrogen bonding with water and to transport battery electrolyte through the coating by diffusion. The presence of the coating on the normally hydrophobic substrate membrane, when used in conjunction with a suitable surfactant, increases the wettability of the substrate membrane and thereby lowers its electrical resistance. The coating also serves to immobilize various soluble electrode derived ions at the coating-electrolyte interface thereby hindering their penetration into the pores of the substrate microporous membrane. Consequently, the plugging of the pores of the substrate membrane by these ions is substantially reduced thereby increasing the life of a battery in which said coated membranes are used as the battery separators.

BACKGROUND OF THE INVENTION

The present invention relates to coated microporous membranes whichexhibit improved wettability and can be used in alkaline storagebatteries and particularly in batteries having electrode systemscontaining zinc and silver electrodes, e.g., nickel-zinc andsilver-zinc, and a process for making the same.

Recent developments in the area of open celled microporous polymericfilms, exemplified by U.S. Pat. Nos. 3,839,516; 3,801,404; 3,679,538;3,558,764; and 3,426,754, have instigated studies to discoverapplications which could exploit the unique properties of these newfilms. Such films which are in effect a gas-breathing water barrier canbe used as vents, gas-liquid transfer mediums, battery separators and avariety of other uses.

One disadvantage of these films, which in the past has limited thenumber of applications to which they may be put, has been theirhydrophobic nature. This is especially true when polyolefinic films, apreferred type of polymeric material often employed in the manufactureof microporous films, are employed. Because these films are not "wetted"with water and aqueous solutions they could not be used advantageouslyin such logical applications as filter media electrochemical separatorcomponents and the like.

Several proposals have been put forth in the past to overcome theseproblems such as exemplified by U.S. Pat. No. 3,853,601; and CanadianPat. No. 981,991, which utilize a variety of hydrophilic surfactantimpregnants. Such surfactant impregnants while imparting hydrophilicityto the microporous film do not maximize the properties of said filmswhen employed as battery separators.

More specifically, a battery separator is a critical component of abattery. A battery is comprised of one or more electrolytic cellsenclosed by a housing. Each cell includes two electrical terminals orelectrodes, the anode and the cathode. The electrodes are immersed in aconducting medium, the electrolyte. Electrical current flows between theelectrodes. This electrical current results from the flow of electronsacross a circuit external to the electrolyte. Just as electrons, flowacross the external circuit so do ions, i.e., charged species, flow inthe electrolyte. Although it is absolutely essential to the productionof an electrical current that ions flow between electrodes in theelectrolyte, it is usually detrimental in a battery for ionic speciesderived from a respective electrode to flow to the electrode of oppositecharge with respect to said ionic species. This interferes with theefficiency of the battery. To prevent deleterious ionic flow of one ormore ionic species between terminals is a function of a batteryseparator. More specifically, a battery separator is disposed in theelectrolytic cell between the anode and cathode of the electrolyte toprevent or retard deleterious ion migration.

The above description suggests the type of material that should ideallybe used as a battery separator. An excellent battery separator is onewhich has pore openings which are small enough to prevent large ionicspecies, such as large electrode derived ions to flow through its poresyet large enough to permit the flow of electrolyte derived ions such asK⁺ and OH⁻ through these pores to reach the electrode of opposite chargein relation thereto. Similarly, the battery separator should be ofminimum thickness in view of the well known fact that the flow of ionsacross a battery separator is inversely proportional to the thickness ofthe separator.

Another requirement of a battery separator suggests itself when oneconsiders that the electrolytes employed in most battery applicationsare highly basic or acidic. A good battery separator should be inert, tothese highly corrosive materials.

Still another requirement for a good battery separator is that theseparator be rapidly wetted by the electrolyte employed. In view of thefact that essentially all of the electrolytes currently utilized areaqueous solutions, this requirement necessitates that the batteryseparator be hydrophilic. The battery separator must be totally andrapidly wetted so as to provide a continuous ionic path on either sideof the battery separator to permit the flow of certain ionstherethrough. An analogy can be drawn to an electrically conductingwire. A break in the wire cuts off the flow of electrons. So, in thecase of an electrolyte, the non-wetting of a portion of a batteryseparator effectively cuts off the path for ionic flow over thenon-wetted area, thus, cutting down on the output of the battery.

Seemingly, the aforedescribed requirements, if satisfied, should beenough to produce a satisfactory battery separator. Unfortunately, inaddition to the above-described criteria for battery separators,additional properties should also be possessed by the same depending onthe type of electrode system employed therein. For example, while anickel-zinc battery has one of the best initial energy-to-weight andpower-to-weight characteristics of known batteries, the same exhibitspoor cycle life, i.e., the number of charge and discharge cycles which abattery can undergo before it no longer is capable of performing itsintended function.

The poor cycle life of nickel-zinc batteries is especially troublesomefor a variety of reasons. This problem is associated with any secondarybattery which employs zinc as the anode and an alkaline electrolyte,because of the high solubility of the oxidation products thereof,namely, ZnO or Zn(OH)₂.

The short cycle life of batteries employing zinc anodes is attributed topremature cell failures which can be characterized as being catastrophicor gradual. Catastrophic cell failures are believed to be due tointernal shorting of the cell by the growth of zinc dendrites which forma bridge between the electrodes.

For example, the nickel-zinc battery is based on the following half-cellreactions:

    2 NiO(OH)⃡2 Ni(OH).sub.2 +2H.sub.2 O+2OH.sup.- -2e.sup.-

    Zn⃡ZnO+H.sub.2 O+2OH.sup.- +2e.sup.-

The reversible reactions are written so that the discharge cycle readsfrom left to right. The zinc half-cell reaction as written above,however, is an oversimplification since the oxidized form of zinc existsas a mixture of ZnO, Zn(OH)₂ and Zn(OH).sup.═₄. The zincate ion(Zn(OH).sup.═₄) is soluble and contributes to the complexity of cellperformance.

When a battery employing a zinc anode is charged, the above-describedreaction reverses and zinc is formed. Ideally, the zinc which is formedis redeposited on the zinc anode. However, some of the zinc which isproduced in a charging sequence characterized by a high current densitygives rise to formation of zinc dendrites which tend to bridge out fromthe zinc anode and connect up with the cathode. Even when a batteryseparator is inserted between the electrodes the zinc dendrites canactually penetrate the separator over a number of charging cyclesleading to catastrophic cell failure.

The gradual, but unacceptable rapid loss of cell energy capacity occursmore frequently with repeated deep discharge cycling wherein the activemass of zinc anode is almost completely depleted. This gradual loss ofenergy capacity is related to pore plugging, other deterioration in theseparator, and to shape change in the zinc electrode. Pore plugging iscaused by the precipitation of various soluble zincate ions (e.g.,Zn(OH).sup.═₄) which are formed during discharge but which becomeinsoluble when the load is removed from the battery. If the solublezincate ions are within the pores when precipitation occurs, the poresbecome plugged thereby reducing the efficiency of the separator.

The shape change of the electrode results from the fact that the zinc isnot redeposited during charging at the location where it has beenoxidized during discharging but is redeposited and concentrated insteadin that part of the cell where the current density is greatest. Thisuneven redepositing of the zinc ions causes densification of theelectrode and reduces its effective surface area. The uneven buildup ofthe zinc causes the electrode to swell in thickness.

The harmful effects of the shape change in the electrode are furtheraggravated when the separator also changes shape by swelling in threedimensions. The combined shape change of the electrode and the separatorcreates pressures in the latter which can rupture the same resulting incell failure.

Many attempts have been made to prevent the formation of dentritic zincand electrode shape change or to avoid the damaging consequencesthereof. Thus, some success has been achieved with a pulsating chargingcurrent, electrolyte additives, electrolyte circulation, and the use ofspecial separators.

More specifically, a great deal of attention is being given to thedesign of battery separators.

As may be gleaned from the above discussion, separator performance isone of the keys to the durability of secondary batteries, particularlyzinc electrode containing batteries. The separator's ability to controlthe flow of electrolyte components plays a limiting role in determiningmaximum power to weight ratio, in maintaining a uniform zinc electrodeshape, and in retarding the diffusion of certain ions, e.g., zincate tothe cathode. Furthermore, initial electrolyte flow properties should notbe altered by the accumulation of ZnO within the pores of the separator.Moreover, the separator is expected to survive the harsh oxidativealkaline environment of the electrolyte in the vicinity of the cathodefor the target life of the cell.

Battery separators employed in the past can be segregated into two basiccategories, namely, those which are diffusion limited and those whichare limited by a mass transport mechanism.

A diffusion limited membrane as defined herein is one in which theexchange of soluble ionic species between one side of the membrane andthe other, which occurs when the membrane is employed as a batteryseparator, does so as a result of the affinity of said ionic species forthe membrane and the rate of said exchange is limited by theconcentration gradient of said soluble ionic species which exists in thesolution present at each side of the membrane for a given membranethickness. The affinity of the ionic species for the membrane is similarto the affinity which a solute has for a solvent.

For example, cellulosic films (e.g., cellophane, sausage casing) havebeen the most common diffusion limited membranes employed as separatorsfor a nickel-zinc cell. When such films are immersed in an electrolytesuch as an aqueous solution of KOH they will absorb electrolyte andwater. It is believed that the exchange of further electrolyte ionsthrough the film occurs by a continuous dynamic process of intermittentattraction between the film and the electrolyte ions. During the courseof this intermittent attraction the electrolyte ions exchange one sitein the film for another and gradually make their way through theinterior of the film until they reach the other side thereof. The poresize of a diffusion limited membrane is within the order of themolecular dimensions of the electrolyte and is therefore too small topermit a free or viscous flow of ions therethrough when an externalforce or pressure is applied to the electrolyte solution to force itthrough the film. It is only by increasing the concentration gradientthat the rate of exchange can be increased. Thus, while the ionicexchange between the planar surfaces of the membrane is also limited bythe thickness of the membrane, and the solubility of the ionic species,when these parameters are fixed it is the limitation on the rate ofexchange of ionic species imposed by the concentration gradient whichparticularly characterizes diffusion limited membranes for purposes ofthe present invention. The dependence of the ionic exchange rate ofdiffusion limited membranes on the concentration gradient isdisadvantageous since there is a limit to the concentration gradientwhich can be employed under high current density and in a real cellenvironment.

This necessarily limits the rate of ionic exchange which in turn limitscharging and discharging rates of batteries which employ diffusionlimited membranes as separators. While the disadvantages of diffusionlimited membranes with respect to the functional relationship betweenrate of ionic exchange and concentration gradient can be compensated forto some extent by reducing the thickness of the membrane, this remedy isimpractical since such membranes must possess a substantial thickness toexhibit sufficient mechanical strength and structural integritynecessary for handling and cell manufacture. Thus, the use of diffusionlimited membranes as a battery separator imposes inherent limitations oncharging and discharging rates on batteries employing the same. If thecharging of discharging rate is too fast, local depletions of hydroxideion occur, leading to electro-osmotic pumping and convective flow ofelectrolyte, which cause erosion and lateral shape changes on the zincelectrodes. Furthermore, cellulosic films are dimensionally unstableleading to rupture for the reasons noted above.

A mass transport limited membrane achieves exchange of ionic species notonly by a diffusion mechanism (due to the presence of a concentrationgradient) but also by actual transport of the ionic species throughporous channels which are large enough to permit unimpeded viscous flowof the ions therethrough. Consequently, a mass transport limitedmembrane is limited the pore volume and thickness of the film. Theexchange of ionic species between the planar surfaces of a masstransport limited membrane is much faster than would otherwise occur ina diffusion limited membrane. Consequently, the inherent limitations oncharging and discharging rates imposed by the use of a diffusion limitedmembrane as a battery separator are absent. A mass transport limitedmembrane is therefore characterized by the ability to increase the flowof a liquid, such as an electrolyte, therethrough in response to anincrease in pressure applied to one side of the membrane. A diffusionlimited membrane will not exhibit this response without rupturing. Forexample, a microporous film prepared in accordance with Example 1,herein, will exhibit a flow of ethanol therethrough of 0.05 cc/cm² /min.at a pressure drop of 760 mmHg while cellophane exhibits substantiallyno measurable flow of the same at the same pressure.

Aside from the aforedescribed limitations associated with cellulosicseparators as a result of their being diffusion limited, the mostlimiting shortcoming of these separators is their degradation in thecell environment. Oxidation of the cellulose within the cell results inthe formation of CO₂ as one of the products of oxidation which reactswith the electrolyte cation, such as potassium, forming for examplepotassium carbonate. The potassium carbonate increases the internalresistance of the cell. Since the CO₂ formation is a manifestation ofthe degradation of the membrane, the membrane can rupture permittingtransfer of oxygen gas formed at the positive electrode uponovercharging thereby lowering of the cell capacity, inducing loss ofnegative electrode capacity, and increasing the risk of thermal runaway.Eventually the physical failure of the degraded cellulosic separatorterminates the cell's life.

Various approaches used to cope with the degradation problem all involvecompromises of cell characteristics and/or cost. For example,electrolyte concentrations above 40% KOH are used with cellulosicseparators to reduce the degradation rate. However, at 31% KOH, wherethe cell's internal resistance would be the lowest, the degradation rateof cellophane is unacceptable.

Multiple layers of cellulose separators permit additional cycles, but atincreased separator cost and weight gain, and an increase in internalresistance. Furthermore, due to inherent swelling characteristics ofcellulose separator films it is difficult to pack several of such filmsin a space efficient fashion.

For example, U.S. Pat. No. 3,894,889 (see also U.S. Pat. No. 3,980,497)is directed to a process for preparing a laminated separator. In oneembodiment the laminated separator comprises two bibulous,non-membranous separator layers, e.g., high grade microporous cellulosicfilter papers, laminated together with a layer of gelling agent, such ascellulose acetate. In a second embodiment a semi-permeable membrane suchas polyethylene is sandwiched between two gel coated bibulous layers.The sandwiched laminate is then hot pressed to form an integral smoothseparator. The resulting laminate structures allegedly provide strengthand protection to the semi-permeable membrane layer during battery cellassembly and operation. Thus, the gelling agent is used merely as a glueand cellulose acetate is not suggested as a means for renderinghydrophobic microporous open-celled membranes hydrophilic or forreducing pore plugging which occurs for example in a nickel-zinc cell.The laminate also results in an undesired weight pain in the separatorand because of the thickness of the laminate separator it cannot bepacked in a minimum of space. Moreover, the aforenoted laminatedseparator does not overcome the basic problem of degradation of themicroporous cellulosic filter papers.

Microporous polypropylene which has a pore size in the order of 200 A isan example of a mass transport limited separator wherein the electrolytebalance is maintained by mass transport thereof through the pores.Because of the ease of electrolyte transport, concentration gradients donot build up during high rate charge and discharge, and convective flowsand electroosmotic pumping effects are reduced. Furthermore,polypropylene is chemically inert in the cell environment, thuspermitting operation at KOH concentrations favoring minimum cellinternal resistance. Such mass transport limited films are not withouttheir own disadvantages, however. For example, the pore structure ofcertain microporous films permits the transfer of zincate to the nickelcompartment. As described above, after repeated cycling, zinc and zincoxide accumulate in the separator. Furthermore, such microporous filmscan be penetrated by zinc dendrites which leads to catastrophic failureof the cell.

Attempts to circumvent the dendrite shorting problem using metal barrierlayers are illustrated in U.S. Pat. Nos. 3,539,374; 3,539,396;3,970,472; 4,039,729.

None of the above patents employs a polymer (e.g., cellulose acetate)coated microporous membrane to reduce dendrite shorting.

U.S. Pat. No. 1,172,183 describes an alkaline primary cell which employstwo separator layers one being microporous propylene known as Celgard™and disposed on top of this layer is one or more non-fibrous cellulosemembranes such as cellophane. While the thickness of the cellophanelayer is not disclosed, the fact that it is initially employed as anintegral film suggests that its thickness is relatively substantial forhandling purposes. Consequently, the double layer separator is distinctfrom the coated microporous films of the subject invention wherein thecoating is substantially thinner than can be achieved using a separatecellophane film. Furthermore, there is no mention of the ability ofcellophane to improve the wetting characteristics of the Celgard™microporous film.

The search has therefore continued for a means of rendering normallyhydrophobic microporous films highly hydrophilic and at the same timeimproving their capacity to act as a battery separator in primary andsecondary batteries. The present invention was developed in response tothis search.

It is therefore an object of the present invention to provide ahydrophilic microporous membrane which is rapidly wettable in aqueous,preferably alkaline aqueous solutions.

It is another object of the present invention to provide a process forrendering a normally hydrophobic microporous membrane hydrophilic.

It is still another object of the present invention to provide ahydrophilic microporous membrane which is substantially dimensionallystable in an alkaline solution.

It is a further object of the present invention to provide a hydrophilicmicroporous membrane which reduces the plugging of its pores byelectrode derived ions, such as by zinc oxide, when employed as abattery separator for a nickel-zinc battery.

It is still another object of the present invention to provide ahydrophilic microporous membrane which retards the migration of silverions therethrough when employed as a battery separator in a silver-zincbattery.

It is another object of the present invention to provide a secondaryelectrochemical cell containing a hydrophilic microporous film batteryseparator.

These and other objects and features of the invention will becomeapparent from the claims and from the following description of thepreferred embodiments of the present invention.

SUMMARY OF THE INVENTION

In one aspect of the present invention there is provided a hydrophilicopen-celled microporous membrane which comprises a normally hydrophobicmicroporous membane, having deposited on at least one surface thereof acoating comprising a polymer selected from the group consisting ofcellulose esters and polyvinyl alcohol, said coating having a drythickness of from about 1 to about 25 microns, and a surfactant disposedwithin said coated microporous membrane in a manner and in at least anamount sufficient to render the substrate microporous membranehydrophilic.

In another aspect of the present invention there is provided a batteryseparator which comprises at least one of the aforedescribed coatedmicroporous membranes.

In still a further aspect of the present invention there is provided animproved battery which employs the aforedescribed coated microporousmembrane as a battery separator.

In still another aspect of the present invention there is provided aprocess for rendering a normally hydrophobic microporous membranehydrophilic and reducing the electrical resistance thereof whichcomprises applying a coating to the surface of said membrane in a mannersufficient to achieve a coating thickness when dry of from about 1 toabout 25 microns, said coating being selected from the group consistingof cellulose esters and polyvinyl alcohol; and impregnating saidmembrane with a surfactant in at least an amount and in a mannersufficient to render the membrane hydrophilic in the absence of saidcoating.

In a still further aspect of the present invention there is provided aprocess for reducing the penetration, of a battery separator which isdisposed between a zinc anode and a cathode which constitute theelectrodes of at least one rechargeable electrolytic cell employing analkaline electrolyte, by electrode derived ions which comprisesemploying as the battery separator at least one of the aforedescribedcoated microporous membranes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In its broadest aspect the present invention is directed to amicroporous membrane which has deposited thereon a thin coating of apolymer, such as cellulose acetate, on one or both sides thereof. Thepolymer coated membrane also has a suitable surfactant impregnated in atleast the pores of the microporous substrate membrane itself andpreferably also in the polymer coating itself.

The polymer coated film is beneficially employed as a separator inalkaline electrolyte primary or secondary battery cells.

The use of the polymer coating in conjunction with a surfactant, yieldsseveral advantages as a battery separator. By way of explanation andwithout wishing to be bound by any particular theory it is believed thata coating polymer, which possesses polar functional groups in thepresence of an alkaline electrolyte, creates a boundary layer at thecoating-electrolyte interface rich in polar functional groups. Forexample, when the polymer coating is cellulose acetate it hydrolyzes inthe presence of alkaline electrolyte to convert the ester groups to freehydroxyl groups and acetic acid. The presence of the hydroxyl groupscreates a hydroxyl rich boundary layer at the film-electrolyteinterface. This polar boundary layer, e.g., hydroxyl rich boundarylayer, exerts two important effects. The first effect is producedregardless of the type of battery electrode system in which theseparator is employed, namely, it facilitates the absorption anddiffusion of the electrolyte by and into the microporous membrane (whichhas been rendered hydrophilic by a surfactant as described herein)thereby substantially improving its hydrophilicity relative tomicroporous films impregnated or coated with equivalent amounts ofconventional surfactants. This improvement in hydrophilicity ismanifested by a reduction in the electrical resistance of the coatedmembrane and by an increase in the degree of absorbtion of theelectrolyte by the substrate membrane. An additional advantage of thepolymer coating is the creation of a barrier which helps to retain thesurfactant within the coated membrane and thereby reduces the loss ofsurfactant which can occur when the dry membrane is stored for longperiods of time. The improvement in electrical resistance (i.e.,reduction) is unexpected because the polymer coating layer is notbelieved to be porous in the sense that a microporous film is.Consequently, the electrolyte passes through the polymer coating by adiffusion mechanism and once inside the film it moves by mass transport.However, because the polymer coating is not employed as a separateintegral film (which film must posses sufficient mechanical strength towithstand the stress of handling and separator manufacture) the polymercoating thickness can be reduced substantially in comparison to anintegral film thereof. Consequently, the increase in hydrophilicity ofthe film induced by the polymer coating reduces the electricalresistance of the coated membrane more than the diffusion barriercreated by the coating increases the same and the resultant separatorpossesses an overall balance of beneficial properties.

The use of a surfactant in the manner described herein is required to atleast render the substrate membrane hydrophilic. The microporoussubstrate membrane is normally hydrophobic and the beneficial effect ofthe hydroxyl rich boundry layer with respect to electrolyte absorptionand diffusion is lost unless a surfactant is eventually disposed withinthe pores of the substrate membrane in such a manner as to render thesame hydrophilic in the absence of the polymer coating. Once thisminimum threshold of hydrophilicity has been imparted to the substratemembrane the polymer coating will improve the hydrophilicity of themembrane even further. Additional beneficial results can be achieveddepending on the mode of application of the surfactant to the membraneas discussed hereinafter.

A second important beneficial effect of the coated membrane separator isobserved depending on the type of electrode system employed in thebattery. Nickel-zinc batteries as described herein cause plugging of thepores of the microporous separators by the precipitation of zincate ionsas zinc oxide from the electrolyte solution when the discharging currentis turned off. The coating on the substrate separator membrane, however,possesses a strong affinity for the soluble zincate ions and immobilizesthem in solution at the coating-electrolyte interface before theypenetrate the substrate membrane on which the coating is deposited.Since the penetration of the microporous membrane by zincate ions issubstantially reduced pore plugging of the same is also substantiallyreduced.

While integral cellophane films, if used as a battery separator (i.e.,regenerated cellulose), will also perform a similar function withrespect to zincate ions, such films are much more dimensionally unstablethan the coating on the microporous membrane of the present invention.For example, cellophane films swell substantially in three dimensions,namely, thickness, length and width when placed in an aqueous alkalinesolution. In contrast, it has been found that the coating of the subjectinvention swells slightly and apparently only in one dimension, namely,thickness. The reasons for the difference in dimensional stabilitybetween the cellophane film and the subject polymer coating is notcompletely understood but is believed to be due in part to the tightbond which develops between the coating and the substrate membrane andin part to the thinness of the polymer coating.

The aforedescribed dimensional stability is particularly advantageousbecause it substantially reduces the danger that shape changes in theelectrode will be magnified by three dimensional changes in the coatedmembrane to the extent that the electrode is damaged. Since themicroporous membrane itself is dimensionally stable and only slightswelling in thickness occurs when the coated separator is contacted withaqueous alkaline electrolyte, several coated membranes can be stacked ina much smaller space than separators which comprise alternating layersof microporous and cellophane films. The use of a separator whichcomprises several layers of efficiently compactable coated membraneshelps to offset the consequences commonly associated with the poorchemical stability and resultant degradation of cellulose based films inan aqueous alkaline environment. The ability to use a multilayeredseparator efficiently reduces the chances that pinpoint degradation ofthe polymer coating will occur at similar locations on each polymercoating layer to provide an unrestricted path of the zincate ions and/orzinc dendrites.

In a silver-zinc cell both the silver and zinc electrodes are soluble incontrast to a nickel-zinc cell wherein only the zinc electrode issoluble. Consequently, the polymer coated membrane in a silver-zinc cellnot only ties up the soluble zincate ions as described above but alsoreacts with soluble silver ions in the electrolyte and thereby retardsthe flow of zinc ions in one direction and the flow of silver ions inthe other direction.

A still further advantage of the cellulose acetate coated membrane isderived from the fact that some of the cellulose acetate becomesdissolved in the electrolyte as oxidized cellulose upon overchargingwhich is known to have a beneficial effect on maintaining cell capacity.(See for example Alkaline Storage Batteries, by Falk and Salkind p. 622(1969) published by Wilely and Sons.)

In view of the above, the preferred form of the aforedescribed membraneis a mass transport limited open-celled microporous film.

Porous or cellular films can be classified into two general types: onetype in which the pores are not interconnected, i.e., a close-celledfilm, and the second type in which the pores are essentiallyinterconnected through tortuous paths which extend from one exteriorsurface or surface region to another, i.e., an open-celled film. Thepreferred porous films employed as the mass transport limited membranein the present invention are of the latter type.

Further, the pores of the porous films employable as the membranesubstrates of the present invention are microscopic, i.e., the detailsof their pore configuration or arrangement are discernible only bymicroscopic examination. In fact, the open-celled pores in the filmsgenerally are smaller than those which can be measured using an ordinarylight microscope, because the wave length of visible light, which isabout 5,000 A (an Angstrom is one ten-billionth of a meter), is longerthan the longest planar or surface dimension of the open cell or pore.The microporous film substrates of the present invention may beidentified, however, by using electron microscopy techniques which arecapable of solving details of pore structure below 5,000 A.

The microporous film substrates of the present invention are alsocharacterized by a reduced bulk density, sometimes hereinafter referredto simply as a "low" density. That is, these microporous films have abulk or overall density lower than the bulk density of correspondingfilms composed of identical polymeric material but having no open-celledor other voidy structure. The term "bulk density" as used herein meansthe weight per unit of gross or geometric volume of the film where grossvolume is determined by immersing a known weight of the film in a vesselpartly filled with mercury at 25° C. and atmospheric pressure. Thevolumetric rise in the level of mercury is a direct measure of the grossvolume. This method is known as the mercury volumenometer method, and isdescribed in the Encyclopedia of Chemical Technology, Vol. 4, page 892(Interscience 1949).

Mass transport limited porous films have been produced which possess amicroporous, open-celled structure, and which are also characterized bya reduced bulk density. Films possessing this microporous structure aredescribed, for example, in U.S. Pat. No. 3,426,754 which patent isassigned to the assignee of the present invention and hereinincorporated by reference. The preferred method of preparation describedtherein involves drawing or stretching at ambient temperatures, i.e.,"cold drawing", a crystalline, elastic precursor film in an amount ofabout 10 to 300% of its original length, with subsequent stabilizationby heat setting of the drawn film under a tension such that the film isnot free to shrink or can shrink only to a limited extent. Other methodsof preparing microporous film are exemplified by U.S. Pat. Nos.3,558,764; 3,843,762; 3,920,785; British Pat. Nos. 1,180,066 and1,198,695 which are all herein incorporated by reference.

While all of the above-listed patents describe processes for preparingmicroporous films, the preferred microporous films are provided inaccordance with the processes described in U.S. Pat. No. 3,801,404 whichdefines a method herein referred to as the "dry stretch" method and U.S.Pat. No. 3,839,516 which defines a method for preparing microporousfilms herein referred to as the "solvent stretch" method, both of whichare herein incorporated by reference. Each of these patents disclosespreferred alternative routes for obtaining a microporous film bymanipulating a precursor film in accordance with specifically definedprocess steps.

The preferred precursor films which may be utilized to preparemicroporous films in accordance with the "dry stretch" and "solventstretch" methods are specifically detailed in each of the aboverespective patents. Thus, the "dry stretch" method utilizes a non-porouscrystalline, elastic, polymer film having an elastic recovery at zerorecovery time (hereinafter defined) when subjected to a standard strain(extension) of 50% at 25° C. and 65 percent relative humidity of atleast 40%, preferably at least about 50% and most preferably at leastabout 80%.

Elastic recovery (ER) as used herein is a measure of the ability of astructure or shaped article such as a film to return to its originalsize after being stretched, and may be calculated as follows: ##EQU1##

Although a standard strain of 50% is used to identify the elasticproperties of the starting films, such strain is merely exemplary. Ingeneral, such starting films will have elastic recoveries higher atstrains less than 50%, and somewhat lower at strains substantiallyhigher than 50%, as compared to their elastic recovery at a 50% strain.

These starting elastic films will also have a percent crystallinity ofat least 20%, preferably at least 30%, and most preferably at least 50%,e.g., about 50 to 90%, or more. Percent crystallinity is determined bythe X-ray method described by R. G. Quynn et al in the Journal ofApplied Polymer Science, Vol. 2, No. 5, pp. 166-173 (1959). For adetailed discussion of crystallinity and its significance in polymers,see Polymers and Resins, Golding (D. Van Nostrand, 1959).

Other elastic films considered suitable for preparing precursor filmsutilized in the "dry stretch" method are described in British Pat. No.1,052,550, published Dec. 21, 1966.

The precursor elastic film utilized in the preparation of themicroporous films by the "dry stretch" process route should bedifferentiated from films formed from classical elastomers such as thenatural and synthetic rubbers. With such classical elastomers thestress-strain behavior, and particularly the stress-temperaturerelationship, is governed by entropy-mechanism of deformation (rubberelasticity). The positive temperature coefficient of the retractiveforce, i.e., decreasing stress with decreasing temperature and completeloss of elastic properties at the glass transition temperature, areparticular consequences of entropy-elasticity. The elasticity of theprecursor elastic films utilized herein, on the other hand, is of adifferent nature. In qualitative thermodynamic experiments with theseelastic precursor films, increasing stress with decreasing temperature(negative temperature coefficient) may be interpreted to mean that theelasticity of these materials is not governed by entropy effects butdependent upon an energy team. More significantly, the "dry stretch"precursor elastic films have been found to retain their stretchproperties at temperatures where normal entropy-elasticity could nolonger be operative. Thus, the stretch mechanism of the "dry stretch"precursor elastic films is thought to be based on energy-elasticityrelationships, and these elastic films may then be referred to as"non-classical" elastomers.

Alternatively, the "solvent stretch" method utilizes a precursor filmwhich must contain at least two components, e.g., an amorphous componentand a crystalline component. Thus, crystalline materials, which are bynature two components, work well with the process. The degree ofcrystallinity of the precursor film must therefore be at least 30%,preferably at least 40% and most preferably at least 50% by volume ofthe precursor film.

The polymers, i.e., synthetic resinous material from which the precursorfilms utilized in either process in accordance with the presentinvention include the olefin polymers, such as polyethylene,polypropylene, poly-3-methyl butene-1, poly-4-methyl pentene-1, 4-methylpentene-1, or ethylene with each other or with minor amounts of otherolefins, e.g., copolymers of propylene and ethylene, copolymers of amajor amount of 3-methyl butene-1 and a minor amount of a straight chainn-alkene such as n-octene-1, n-hexadecene-1, n-octadecene-1, or otherrelatively long chain alkenes, as well as copolymers of 3-methylpentene-1 and any of the same n-alkenes mentioned previously inconnection with 3-methyl butene-1.

For example, in general when propylene homopolymers are contemplated foruse in the "dry stretch" method, an isotactic polypropylene having apercent crystallinity as indicated above, a weight average molecularweight ranging from about 100,000 to 750,000 (e.g., about 200,000 to500,000) and a melt index (ASTM-D-1238-57T, Part 9, page 38) from about0.1 to about 75, (e.g., from 0.5 to 30), can be employed so as to give afinal film product having the requisite physical properties.

It is to be understood that the terms "olefinic polymer" and "olefinpolymer" are used interchangeably and are intended to describe a polymerprepared by polymerizing olefin monomers through their unsaturation.

Preferred polymers for use in the "solvent stretch" method are thosepolymers utilized in accordance with the invention described in U.S.patent application Ser. No. 44,805, filed June 2, 1979, by John W.Soehngen and assigned to the Assignee of the present invention, entitled"Improved Solvent Stretch Process for Preparing Microporous Films fromPrecursor Films of Controlled Crystalline Structure" the disclosure ofwhich is herein incorporated by reference. Thus, a polyethylenehomopolymer having a density of from about 0.960 to about 0.965 gm/cc, ahigh melt index of not less than about 3 and preferably from about 3 toabout 20 and a broad molecular weight distribution ratio (M_(w) /M_(n))of not less than about 3.8 and preferably from about 3.8 to about 13 ispreferred in preparing a microporous film by the "solvent stretch"method. Moreover, nucleating agents may be incorporated into the polymeremployed to prepare the precursor film as described in the incorporatedSoehngen application in which case the polymers having a melt index aslow as 0.3 may be employed.

The types of apparatus suitable for forming the precursor films are wellknown in the art.

For example, a conventional film extruder equipped with a shallowchannel metering screw and a coat hanger die is satisfactory. Generally,the resin is introduced into a hopper of the extruder which contains ascrew and a jacket fitted with heating elements. The resin is melted andtransferred by the screw to the die from which it is extruded through aslit in the form of a film from which it is drawn by a take-up orcasting roll. More than one take-up roll in various combinations orstages may be used. The die opening or slit width may be in the range,for example, of about 10 to 200 mils.

Using this type of apparatus, film may be extruded at a drawdown ratioof about 5:1 to 200:1, preferably 10:1 to 50:1.

The terms "drawdown ratio" or more simply, "draw ratio", as used hereinis the ratio of the film wind-up or take-up speed to the speed of thefilm issuing at the extrusion die.

The melt temperature for film extrusion is, in general, no higher thanabout 100° C. above the melting point of the polymer and no longer thanabout 10° C. above the melting point of the polymer.

For example, polypropylene may be extruded at a melt temperature ofabout 180° C. to 270° C., preferably 200° C. to 240° C. Polyethylene maybe extruded at a melt temperature of about 175° C. to 225° C.

When the precursor film is to be utilized in accordance with the "drystretch" method, the extrusion operation is carried out with rapidcooling and rapid drawdown in order to obtain maximum elasticity anddevelop a row lamella structure. This may be accomplished by having thetake-up roll relatively close to the extrusion slit, e.g., within twoinches and, preferably, within one inch. An "air knife" operating attemperatures between, for example, 0° C. and 40° C., may be employedwithin one inch of the slit to quench, i.e., quickly cool and solidify,the film. The take-up roll may be rotated, for example, at a speed of 10to 1000 ft/min, preferably 50 to 500 ft/min.

When the precursor film is to be utilized in accordance with the"solvent stretch" method, the extrusion operation is preferably carriedout with slow cooling, in order to minimize stress and any associatedorientation which might result from a fast quench to obtain maximumcrystallinity and numerous small spherulites but yet fast enough toavoid developing large spherulites. This may be accomplished bycontrolling the distance of the chill roll take-up from the extrusionslit.

While the above description has been directed to slit die extrusionmethods, an alternative method of forming the precursor filmscontemplated in this invention is the blown film extrusion methodwherein a hopper and an extruder are employed which are substantiallythe same as in the slit extruder described above.

From the extruder, the melt enters a die from which it is extrudedthrough a circular slit to form a tubular film having an initialdiameter D₁. Air enters the system through an inlet into the interior ofsaid tubular film and has the effect of blowing up the diameter of thetubular film to a diameter D₂. Means such as air rings may also beprovided for directing the air about the exterior of extruded tubularfilm so as to provide different cooling rates. Means such as a coolingmandrel may be used to cool the interior of the tubular film. After adistance during which the film is allowed to completely cool and harden,it is wound up on a take-up roll.

Using the blown film method, the drawdown ratio is preferably 5:1 to100:1, the slit opening 10 to 200 mils, preferably 40 to 100 mils, theD₂ /D₁ ratio, for example, 1.0 to 4.0 and preferably about 1.0 to 2.5,and the take-up speed, for example, 30 to 700 ft/min. The melttemperature may be within the ranges given previously for slit dieextrusion.

The extruded film may then be initially heat treated or annealed inorder to improve crystal structure, e.g., by increasing the size of thecrystallites and removing imperfections therein. Generally, thisannealing is carried out at a temperature in the range of about 5° C. to100° C. below the melting point of the polymer for a period of a fewseconds to several hours, e.g., 5 seconds to 24 hours, and preferablyfrom about 30 seconds to 2 hours. For polypropylene, the preferredannealing temperature is about 100° C. to 155° C.

An exemplary method of carrying out the annealing is by placing theextruded film in a tensioned or tensionless state in an oven at thedesired temperature in which case the residence time is preferably inthe range of about 30 seconds to 1 hour.

In the preferred embodiments, the resulting partly-crystalline precursorfilm is preferably subjected to one of the two alternative proceduresdescribed above to obtain a normally hydrophobic microporous film whichmay be utilized as the membrane substrate in the present invention.

The first preferred procedure as disclosed in U.S. Pat. No. 3,801,404,herein referred to as the "dry stretch" method, includes the steps ofcold stretching, i.e., cold drawing, the elastic film until poroussurface regions or areas which are elongated normal or perpendicular tothe stretch direction are formed, (2) hot stretching, i.e., hot drawing,the cold stretched film until fibrils and pores or open cells which areelongated parallel to the stretch direction are formed, and thereafter(3) heating or heat-setting the resulting porous film under tension,i.e., at substantially constant length, to impart stability to the film.

The term "cold stretching" as used herein is defined as stretching ordrawing a film to greater than its original length and at a stretchingtemperature, i.e., the temperature of the film being stretched, lessthan the temperature at which melting begins when the film is uniformlyheated from a temperature of 25° C. and at a rate of 20° C. per minute.The term "hot stretching" as used herein is defined as stretching abovethe temperature at which melting begins when the film is uniformlyheated from a temperature of 25° C. and at a rate of 20° C. per minute,but below the normal melting point of the polymer, i.e., below thetemperature at which fusion occurs. As is known to those skilled in theart, the temperature at which melting begins and the fusion temperaturemay be determined by a standard differential thermal analyzer (DTA), orby other known apparatus which can detect thermal transitions of apolymer.

The temperature at which melting begins varies with the type of polymer,the molecular weight distribution of the polymer, and the crystallinemorophology of the film. For example, polypropylene elastic film may becold stretched at a temperature below about 120° C. preferably betweenabout 10° C. and 70° C. and conveniently at ambient temperature, e.g.,25° C. The cold stretched polypropylene film may then be hot stretchedat a temperature above about 120° C. and below the fusion temperature,and preferably between about 130° C. and 150° C. Again, the temperatureof the film itself being stretched is referred to herein as the stretchtemperature. The stretching in these two steps or stages must beconsecutive, in the same direction, and in that order, i.e., cold thenhot, but may be done in a continuous, semi-continuous, or batch process,as long as the cold stretched film is not allowed to shrink to anysignificant degree, e.g., less than 5% of its cold stretched length,before being hot stretched.

The sum total amount of stretching in the above two steps may be done inthe range of about 10 to 300% and preferably about 50 to 150%, based onthe initial length of the elastic film. Further, the ratio of the amountof hot stretching to the sum total amount of stretching or drawing maybe from above about 0.10:1 to below 0.99:1, preferably from about 0.50:1to 0.97:1, and most preferably from about 0.50:1 to 0.95:1. Thisrelationship between the "cold" and "hot" stretching is referred toherein as the "extension ratio" (percent "hot" extension to the percent"total" extension).

In any stretching operation where heat must be supplied the film may beheated by moving rolls which may in turn be heated by an electricalresistance method, by passing over a heated plate, through a heatedliquid, a heated gas, or the like.

After the above-described two stage or two step stretching, thestretched film is heat set. This heat treatment may be carried out at atemperature in the range from about 125° C. up to less than the fusiontemperature, and preferably about 130° to 160° C. for polypropylene;from about 75° C. up to less than fusion temperature, and preferablyabout 115° C. to 130° C., for polyethylene, and at similar temperatureranges for other of the above mentioned polymers. This heat treatmentshould be carried out while the film is being held under tension, i.e.,such that the film is not free to shrink or can shrink to only acontrolled extent not greater than about 15% of its stretched length,but not so great a tension as to stretch the film more than anadditional 15%. Preferably, the tension is such that substantially noshrinkage or stretching occurs, e.g., less than 5% change in stretchedlength.

The period of heat treatment which is preferably carried outsequentially with and after the drawing operation, should not be longerthan 0.1 second at the higher annealing temperatures and, in general,may be within the range of about 5 seconds to 1 hour and preferablyabout 1 to 30 minutes.

The above described setting steps may take place in air, or in otheratmospheres such as nitrogen, helium or argon.

A second preferred alternative procedure for converting theaforedescribed respective precursor film to a microporous film asdescribed in U.S. Pat. No. 3,839,516 and herein referred to as the"solvent stretch" method includes the basic steps of (1) contacting theprecursor film having at least two components (e.g., an amorphouscomponent and a crystalline component), one of which is lesser in volumethan all the other components, with a swelling agent for sufficient timeto permit adsorption of the swelling agent into the film; (2) stretchingthe film in at least one direction while in contact with swelling agent;and (3) maintaining the film in its stretched state during removal ofthe swelling agent. Optionally, the film may be stabilized byheat-setting under tension or by ionizing radiation.

Generally, a solvent having a Hildebrand solubility parameter at or nearthat of the polymer would have a solubility suitable for the drawingprocess described herein. The Hildebrand solubility parameter measuresthe cohesive energy density. Thus, the underlying principle relies onthe fact that a solvent with a similar cohesive energy density as apolymer would have a high affinity for that polymer and would beadequate for this process.

General classes of swelling agents from which one appropriate for theparticular polymeric film may be chosen are lower aliphatic ketones suchas acetone, methylethylketone, cyclohexanone; lower aliphatic acidesters such as ethyl formate, butyl acetate, etc.; halogenatedhydrocarbons such as carbon tetrachloride, trichloroethylene,perchloroethylene, chlorobenzene, etc.; hydrocarbons such as heptane,cyclohexane, benzene, xylene, tetraline, decaline, etc.;nitrogen-containing organic compounds such as pyridine, formamide,dimethylformamide, etc.; ethers such as methyl ether, ethyl ether,dioxane, etc. A mixture of two or more of these organic solvents mayalso be used.

It is preferred that the swelling agents be a compound composed ofcarbon, hydrogen, oxygen, nitrogen, halogen, sulfur and contain up toabout 20 carbon atoms, preferably up to about 10 carbon atoms.

The "solvent stretching" step may be conducted at a temperature in therange of from above the freezing point of the swelling agent, to a pointbelow the temperature at which the polymer dissolves (i.e., ambienttemperature to about 50° C.).

The precursor film employed in the "solvent stretch" process may rangefrom about 0.5 to about 5 mils, or even thicker, subject to theelectrical resistance requirements discussed herein.

In an alternative embodiment the precursor film is biaxially stretchedin accordance with the procedures disclosed in U.S. Patent ApplicationSer. No. 44,801, filed on June 1, 1979, entitled "Improved SolventStretching Process for Preparing Microporous Films" and assigned to theassignee of the present invention, the disclosure of which is hereinincorporated by reference. This process identifies preferred stretchingconditions in a uniaxial direction which lead to improved permeabilityof the uniaxially "solvent stretched" microporous film. The uniaxiallystretched microporous film can then be stretched in a transversedirection to improve the permeability even further. Thus, it ispreferred that the precursor film be "solvent stretched" in a uniaxialdirection not greater than about 350%, and most preferably 300% greaterthan its original length. Typically, additional stretching in the samedirection after the solvent removal is not employed.

The optional stabilizing step may be either a heatsetting step or across-linking step. This heat treatment may be carried out at atemperature in the range from about 125° C. up to less than the fusiontemperature and preferably about 130° to 150° C. for polypropylene; fromabout 75° C. up to less than fusion temperature, and preferably about115° to 130° C. for polyethylene and at similar temperature ranges forother of the above mentioned polymers. This heat treatment should becarried out while the film is being held under tension, i.e., such thatthe film is not free to shrink or can shrink to only a controlled extentnot greater than about 15 percent of its stretched length, but not sogreat a tension as to stretch the film more than an additional 15percent of its stretched length. Preferably, the tension is such thatsubstantially no shrinkage or stretching occurs, e.g., less than 5%change in stretched length.

The period of heat treatment which is preferably carried outsequentially with and after the "solvent stretching" operation,shouldn't be longer than 0.1 second at the higher annealing temperaturesand, in general, may be within the range of about 5 seconds to 1 hourand preferably about 1 to 30 minutes.

The above described setting steps may take place in air, or in otheratmospheres such as nitrogen, helium or argon.

When the precursor film is biaxially stretched the stabilizing stepshould be conducted after transverse stretching and not before.

While the present disclosure is directed primarily to the aforesaidolefin polymers in connection with their use in the "dry stretch" or"solvent stretch" procedures, the invention also contemplates the use ofhigh molecular weight acetal, e.g., oxymethylene, polymers to prepareprecursor films which can be rendered microporous as described herein.While both acetal homopolymers and copolymers are contemplated, thepreferred acetal polymer for purposes of polymer stability is a "random"oxymethylene copolymer, which contains recurring oxymethylene, i.e.,--CH₂ --O--, units interspersed with --OR-- groups in the main polymerchain where R is a divalent radical containing at least two carbon atomsdirectly linked to each other and positioned in the chain between thetwo valences, with any substituents on said R radical being inert, thatis, those which do not include interfering functional groups and whichwill not induce undesirable reactions, and where a major amount of the--OR-- units exist as single units attached to oxymethylene groups oneach side. Examples of preferred polymers include copolymers of trioxaneand cyclic ethers, containing at least two adjacent carbon atoms such asthe copolymers disclosed in U.S. Pat. No. 3,027,352 of Walling et al.These polymers in film form may also have a crystallinity of at least 20percent, preferably at least 30 percent, and most preferably at least 50percent, e.g., 50 to 60 percent or higher. Further, these polymers havea melting point of at least 150° C. and a number average molecularweight of at least 10,000. For a more detailed discussion of acetal andoxymethylene polymers, see Formaldehyde, Walter, pp. 175-191, (Reinhold1964).

Other relatively crystalline polymers, from which precursor films can bederived to which the "dry stretch" or solvent stretch methods may beapplied, are the polyalkylene sulfides such as polymethylene sulfide andpolyethylene sulfide, the polyarylene oxides such as polyphenyleneoxide, the polyamides such as polyhexamethylene adipamide (nylon 660)and polycaprolactam (nylon 6), all of which are well known in the artand need not be described further herein for the sake of brevity.

The microporous films produced by the above described methods and whichcan be employed as substrates in the present invention, in a tensionlessstate, have a lowered bulk density compared with the density ofcorresponding polymeric materials having no open-celled structure, e.g.,those from which it is formed. Thus, the films have a bulk density nogreater than about 95% and preferably 20 to 40% of the precursor film.Stated another way, the bulk density is reduced by at least 5% andpreferably 60 to 80%. For polyethylene, the reduction is 30 to 80%,preferably 60 to 80%. The bulk density is about 20 to 40% of thestarting material, the porosity has been increased by 60 to 80% becauseof the pores or holes.

When the microporous film is prepared by the "dry-stretch" or "solventstretch" methods the final crystallinity of the microporous film ispreferably at least 30 percent, more preferably at least 65%, and moresuitably about 70 to 85%, as determined by the X-ray method described byR. G. Quynn et al in the Journal of Applied Polymer Science, Vol. 2, No.5, pp. 166-173. For a detailed discussion of crystallinity and itssignificance in polymers, see Polymers and Resins, Golding (S. VanNostrant, 1959).

The microporous films which can be employed as substrates in the presentinvention and which can be produced by the aforenoted methods may alsohave an average pore size of from about 200 to about 10,000 A, typicallyfrom about 200 to about 5000 A, and more typically about 200 to about400 A. These values can be determined by mercury porosimetry asdescribed in an article by R. G. Quynn et al, on pages 21-34 of TextileResearch Journal, January, 1963 or by the use of electron microscopy asdescribed in Geil's Polymer Single Crystals, p. 69 (Interscience 1963).When an electron micrograph is employed pore length and widthmeasurements can be obtained by simply utilizing a ruler to directlymeasure the length and width of the pores on an electron micrographtaken usually at 2,000 to 50,000 magnification. Generally, the porelength values obtainable by electron microscopy are approximately equalto the pore size values obtained by mercury porosimetry.

The microporous films which can be employed as substrates in the presentinvention will exhibit a surface area within certain predictable limitswhen prepared by either the "solvent stretch" method or the "drystretch" method. Typically such microporous films will be found to havea surface area of at least 10 sq.m/gm and preferably in the range ofabout 15 to about 50 sq.m/gm. For films formed from polyethylene, thesurface area generally ranges from about 10 to about 25 sq.m/gm. andpreferably about 20 sq.m/gm, and for polypropylene from about 20 toabout 50 sq.m/gm.

Surface area may be determined from nitrogen or krypton gas adsorptionisotherms using a method and apparatus described in U.S. Pat. No.3,262,319. The surface area obtained by this method is usually expressedas square meters per gram.

In order to facilitate comparison of various materials, this value canbe multiplied by the bulk density of the material in grams per cc.resulting in a surface area expressed as square meters per cc.

A further characteristic of the microporous membranes which can beutilized in the instant invention is its porosity.

The porosity of the microporous film membranes suitable for use in thepresent invention may be defined as a percent ratio of the total volumeoccupied by the void space of a standard sample of microporous film tothe bulk volume of the same sample which is the sum of the void spacevolume and the volume occupied by the solid material of the film itself.The % porosity is determined by measuring the thickness, length andwidth of a microporous film sample to determine the film's bulk volume.The film is then weighed and the density of the film is determined. Thedensity of the polymer resin used to prepare the film is thendetermined. The % porosity is then calculated from the equation:##EQU2##

The porosity of the microporous film membranes suitable for use in thepresent invention and obtainable from the aforenoted methods may varyfrom about 30 to about 85%, preferably from about 30 to about 45%, andmost preferably from about 35 to about 45%.

The above described microporous polymeric films which can be employed assubstrate membranes in the instant invention have a thickness of fromabout 0.7 (0.001 inch) to about 8 mils, preferably from about 0.7 toabout 4 mils; and most preferably from about 0.7 to about 2 mils (e.g.,1 mil).

In order to be useful as a battery separator the noncoated microporoussubstrate membrane should be capable of exhibiting an electricalresistance of not greater than about 50 milliohms-square inch(milliohms-in²), preferably not greater than about 20 milliohms-in², andmost preferably not greater than about 5 milliohms-in² when renderedhydrophilic as described herein.

Electrical resistance as defined herein is a measure of the ability ofthe microporous membrane to conduct ions. Consequently, as a generalrule the higher the electrical resistance of the microporous film theless effective it will be as a battery separator.

Electrical resistance (direct current method) of a microporous film asdefined herein and employed in the claims is determined by soaking asample thereof having a known surface area (e.g., 0.2 sq. inches) inabout a 40% by weight, solution of KOH in water for 24 hours. Theresulting sample is then disposed between working platinum electrodes(i.e., anode and a cathode) immersed in an electrolyte of a 40%, byweight, solution of KOH in water and a direct current of known amperage(e.g., 40 milliamperes) is passed through the cell between theelectrodes. The potential drop across the film (E') is measured with anelectrometer. The potential drop across the cell without the microporousfilm disposed therein (E) is also determined using the same current. Eis measured in millivolts.

The electrical resistance of the microporous film is then determinedusing the equation: ##EQU3## where A is the surface area of the exposedfilm in square inches, I is the current across the cell in milliamperes,E.R. is the electrical resistance of the microporous film inmilliohms-square inch, and E' and E are as described.

The above described microporous films prepared in accordance theaforenoted "dry stretch" and "solvent stretch" methods will exhibit thelow electrical resistances, are mass transport limited when employed asa battery separator, are resistant to oxidation, and in general possessall of the properties which are deemed necessary to render themcommercially suitable for use as microporous substrates in the presentinvention.

However, it is contemplated that any conventional normally hydrophobicmicroporous substrate membrane commonly used in battery separators whichis mass transport limited and preferably those which possess theaforenoted pore structure, and electrical resistance, regardless of howthey are made or the material from which they are derived may beemployed as the substrate for the barrier material coating.

Furthermore, the substrate membrane to which the polymer coating isapplied may comprise any of the metal coated microporous membranesdescribed in U.S. Patent Application Ser. No. 125,195, filed Feb. 27,1980 entitled "Coated Open-celled Microporous Membranes" by H. Taskierthe disclosure of which is herein incorporated by reference. The polymercoating can be applied to either the metal coated surface, the filmsurface when only one side is metal coated or both.

The present invention not only provides a substrate with uniform andhighly advantageous mass transport properties but because of theuniformity of its overall pore structure it permits the development ofuniform coatings deposited thereon.

While the preferred configuration of the substrate membrane is a film,preferably an embossed film for reasons described hereinafter, thesubstrate membrane may possess other configurations which render itsuitable for its intended end use, such as a battery separator, which iswithin the skill in the art including fibers, tubes (e.g., hollowfibers), bags, and the like.

Alternatively, the microporous substrate membrane may comprise amicroporous film, such as that prepared by the aforenoted "dry stretch"or "solvent stretch" methods which have a non-woven fibrous feltembossed on at least one surface of the same. Such non-woven feltspreferably comprise substantially continuous randomly arrangedfilamentary material, particularly polyolefins such as polypropylene ofvarying crystallinity, and typically of varying diameter, which extendgenerally parallel to the plane of the film, thermally bonded to itselfat randomly located filament crossover points and to the film atrandomly located contact points between the filamentary material and thefilm. Such laminates are preferably prepared by spray spinning and spunbonding techniques or by lamination of a previously formed spun-bondednon-woven web to the microporous membrane. Other suitable filamentarymaterials in addition to polyolefins include cellulose acetate,polyamides, polyacetals, polyalkylene sulfides, and polyarylene oxides.The resulting thermally bonded filamentary layer-microporous filmlaminate is then embossed i.e., pressed between moving embossing rollsor rollers heated to elevated temperatures of about 100° to about 150°C. in a conventional calendering machine. The embossing of the laminateimparts raised or projected design ridges in relief on the surfacethereof which have the advantageous effect of providing gas channels forescape of gases upon overcharge. The embossed laminate also has theadditional advantage of providing a buttress effect for the polymercoating when applied to the fiber containing side thereof which furtherimproves the adhesion of the coating to the substrate membrane.

The thickness of the embossed non-woven filamentary layer typically willvary from about 1 to about 10 mils, preferably from about 1 to about 7mils, and most preferably from about 1 to about 5 mils. The filamentarylayer is preferably deposited to cover an area which is coextensive withthe surface of the substrate membrane on which it is placed.

The filamentary material which is spray spun, and spun-bonded, on thesurface of the microporous film typically has a denier per filament(dpf) of from about 0.5 to about 5, preferably from about 1 to about 4dpf, and most preferably from about 2 to about 3.5 dpf. The thickness ofthe laminate will generally be from about 2 to about 8 mils (e.g., 3mils).

Alternatively, an integral non-woven web can be first prepared, forexample, by a spray spinning technique, and this web then laminated tothe microporous membrane by embossing as described in U.S. Pat. No.3,679,540, the disclosure of which is herein incorporated by reference.

The description and preparation of the microporous film non-wovenfilamentary laminate by spray spinning which can be embossed is providedin U.S. Pat. No. 3,932,682 the disclosure of which is hereinincorporated by reference. A more detailed description of the preferredspray spinning technique used to prepare said laminates or non-wovenfelt webs is provided in U.S. Pat. No. 3,543,332 the disclosure of whichis also herein incorporated by reference.

The coating polymers which are capable of achieving the aforedescribedadvantages when coated on the microporous membrane are those which canabsorb water and swell in an aqueous alkaline environment. Such waterabsorption properties are derived from the presence, in an aqueousalkaline environment, of functional groups such as hydroxyl groups whichthereby impart to the coating the capability of hydrogen bonding withwater and of transporting electrolyte through the coating by diffusion,at least when the coating has a suitable surfactant incorporated (i.e.,uniformly distributed) therein.

Other examples of suitable functional groups include carboxyl,anhydride, and amino groups.

The preferred class of polymers employed in the coating includecellulose esters exemplified by cellulose acetate, cellulose triacetate,cellulose butyrate, propionate and the like, and mixed cellulose estersexemplified by cellulose acetate propionate, and cellulose acetatebutyrate. The most preferred cellulose ester is cellulose acetate.

The preferred cellulose acetate is supplied in the form of secondarycellulose acetate flake containing from about 52 to 56% of combinedacetic acid. Typically, a 20% solution of such cellulose acetate in asolvent comprising 9 parts acetone and 1 part ethanol has a viscosity ofbetween about 21 and 82 seconds when determined by the falling ballmethod (ASTM D 851-56) using a stainless steel ball 3/32 inch indiameter.

Other classes of suitable coating polymers include polyvinyl alcohol.

The polyvinyl alcohol preferably has a number average molecular weightof from about 125,000 to about 135,000, a viscosity at 25° C. in waterof from about 25 to about 45 cps and a degree of hydrolysis of fromabout 85 to about 99.8%.

The surfactant which is employed in conjunction with the coating polymeris employed to achieve at least wetting of the microporous substratemembrane by the electrolyte and works in conjunction with the polymer toimprove the hydrophylicity of the resulting end product.

The surfactant is preferably chosen to be at least partially compatiblewith the coating polymer (i.e., the membrane remains essentially clearafter impregnation) and has the effect of plasticizing the latter tofacilitate coating of the microporous membrane, and also is preferablysoluble or dispersable in the solvent used to prepare the coatingpolymer solution.

Accordingly, any surfactant conventionally employed to rendermicroporous membranes hydrophilic may be employed subject to theaforenoted compatibility requirement.

As used herein the term "hydrophobic" is defined as meaning a surfacewhich passes less than about 0.010 milliliter of water per minute persq. cc. of flat film surface under a water pressure of 100 psi. Likewisethe term "hydrophilic" is meant to be applied to those surfaces whichpass greater than about 0.01 milliliter of water per minute per sq. cc.at the same pressure.

Accordingly, any surfactant which, when applied to the microporoussubstrate membrane alone (i.e., in the absence of the coating polymer),lowers the surface tension thereof to the extent that the substrate willexhibit a contact angle with water of less than about 80°, preferablyless than about 60°, will render said substrate hydrophilic and can beemployed in conjunction with the coating polymer.

Representative examples of suitable preferred surfactants includesilicon glycol copolymers, such as polyoxyethylene polymethyl siloxane,either alone or in combination with an imidazoline tertiary amine asdescribed in U.S. Pat. No. 3,929,509 the disclosure of which is hereinincorporated by reference. Other suitable preferred surfactants includephosphate esters such as ethoxylated 2-ethyl-hexyl phosphate. Alsoincluded are any of the hydrophilic organic hydrocarbon monomersdisclosed in U.S. Patent Application Ser. No. 071,644, filed Sept. 4,1979 entitled "Hydrophilic Monomer Treated Microporous Film" by NelsonLazear such as acrylic acid, methacrylic acid, vinyl acetate andmixtures thereof, which preferably are chemically fixed within themicroporous substrate membrane in accordance with the proceduresdescribed therein. The disclosure of this application is hereinincorporated by reference. Further suitable surfactants include thosedescribed in U.S. Pat. No. 3,472,700 and Canadian Pat. No. 981,991 thedisclosure of which is herein incorporated by reference.

The coating polymer is applied to the surface of the microporousmembrane by mixing the same in a suitable solvent which preferablycontains at least one of the aforenoted surfactants. The concentrationof the polymer in the solvent typically will vary from about 1 to about25% and preferably from about 2 to about 15%, by weight, based on thepolymer solution weight.

The resulting polymer solution is then coated on the surface of themicroporous substrate membrane using any suitable coating means such asroll coating, reverse roll coating, coating by means of wire wound rodsand the like. Upon drying of the coating, a thin coating of the polymeris produced on the surface of the microporous substrate.

Suitable solvents for combining the cellulose ester polymer, andsurfactant include ketones such as acetone, methylethylketone, methylenechloride/methanol mixtures (e.g., 1:1 w/w) and ethers such as ethyleneglycol monomethyl ether also known as methyl "Cellosolve".sup.™.Suitable solvents for the polyvinyl alcohol include water, ethanol,methanol and the like and mixtures thereof. In short, any solvent whichdissolves the coating polymer and will not adversely affect themicroporous membrane when applied to the surface thereof may beemployed.

It is to be understood that the means for applying the coatingcomposition to the surface of the microporous substrate membrane is notrestricted to solutions of volatile organic solvents. Aqueous,dispersions of the polymer and surfactant may also be employed as thevehicle from which the coating is laid down on the surface of themicroporous substrate.

The surfactant can be applied to the microporous substrate membrane byat least two different alternative embodiments or combinations of thesame. In the preferred embodiment the surfactant is mixed with thepolymer coating solution and the mixture is applied to the surface ofthe substrate membrane. Alternatively, the surfactant is impregnatedinto the substrate membrane by conventional means either before or afterthe same is coated with the polymer solution which may or may notcontain additional surfactant. The effects induced by the surfactantwill differ depending on the method of its application.

Regardless of the method of application, the amount of surfactant whichis employed in conjunction with each method is sufficient to at leastrender the substrate membrane hydrophilic in the absence of the polymercoating. To achieve an optimum reduction in electrical resistance,however, more than the minimum amount of surfactant should be employed.The particular amounts employed for each embodiment and the effectsproduced thereby are described hereinafter.

When the surfactant is mixed with the polymer coating it serves twoimportant functions, namely, it facilitates adsorbtion of the alkalineelectrolyte by the coating deposited on the microporous membrane, andmore importantly, when the solvent evaporates through the substratemembrane, the surfactant is carried along therewith, deposited in thepores of the substrate membrane, and thereby renders the samehydrophilic. The use of a surfactant is necessary since the polymercoating is not capable of penetrating the pores of the microporousmembrane and consequently the polymer cannot, by itself, render themicroporous film hydrophilic.

The reason the surfactant is preferably mixed with the coating polymeris to ensure the most efficient use of the same and prevent undue lossthereof which can occur when the coating polymer application vehicle isa solvent for the surfactant. For example, while it is possible toimpregnate the microporous membrane with surfactant and then apply thecoating polymer solution to the film surface, such a sequence can resultin penetration of the film by the solvent of the polymer solutionthereby washing the surfactant out of the film. Similar problems canresult where the substrate membrane is coated with the polymer coatingfirst and then impregnated with a surfactant solution. If the surfactantsolvent vehicle is also a solvent for the polymer coating the latter canbe removed during surfactant impregnation.

Thus, while the preferred embodiment wherein the surfactant and acoating polymer are mixed achieves the most beneficial results describedhereinafter and is believed to represent the most efficient embodiment,beneficial results are also achieved when the surfactant is initiallyapplied to the substrate microporous membrane itself by means other thanvia the coating polymer solution provided the polymer coating is notadversely influenced thereby.

In those embodiments where the surfactant is applied directly to themicroporous membrane any method known in the art for achieving thisobjective (e.g., coating or impregnating) can be employed as describedin U.S. Pat. No. 3,929,509. In this embodiment simply passing themicroporous substrate membrane through a solution of the surfactant isthe preferred method of surfactant application.

The amount of surfactant which is admixed with the polymer coating forapplication to the substrate membrane will vary depending on theidentity of the surfactant and can be any amount which is effective to(a) render the microporous substrate membrane hydrophilic as definedherein so that the electrolyte can pass through the substrate membraneafter it diffuses through the polymer coating, and preferably alsoeffective to (b) facilitate and increase the rate of absorption of thealkaline electrolyte by the polymer coating and thereby to facilitatediffusion of said electrolyte through the coating into the substratemembrane. Accordingly, in order to achieve the minimum acceptable effectsuch amounts must at least be effective to provide sufficient surfactantto enable it to pass into the microporous substrate membrane asdescribed herein. Thus, while any effective amount of the surfactant maybe mixed with the polymer it is preferred that such amounts constitutefrom about 20 to about 200%, preferably from about 50 to about 70%, andmost preferably from about 100 to about 150%, by weight, based on theweight of the coating polymer.

When the surfactant is applied directly to the substrate membraneitself, it is impregnated and deposted in the pores of the same inamounts which would at least render the uncoated substrate membranehydrophilic as described herein. Accordingly, in this embodiment whileany effective amount of surfactant may be impregnated into themicroporous substrate membrane it is preferred that such amountconstitute from about 8 to about 40%, preferably from about 10 to about30%, and most preferably from about 20 to about 30%, by weight, based onthe weight of the uncoated substrate membrane. Such amounts thereforealso define the amount of surfactant which preferably should eventuallypass into the pores of the substrate membrane to render it hydrophilicwhen the same is applied via the preferred embodiment, namely, by thepolymer coating.

In both of the aforenoted embodiments, while a surfactant impregnatedsubstrate membrane alone is considered to be hydrophilic, the presenceof the polymer coating thereon improves the membrane's hydrophilicnature even further as evidenced by a reduction in electrical resistancein relation to the uncoated film impregnated with an equivalent amountof surfactant.

The thickness of the polymer coating is governed by the desire toincrease the hydrophilicity of the uncoated microporous substratemembrane as much as possible and at the same time to reduce plugging ofthe pores of the substrate membrane by electode derived ions when thesame is employed as a battery separator.

Accordingly, the dry thickness of the polymer coating on any one surfaceof the substrate membrane is controlled to be from about 1 to about 25microns (e.g., 1 to about 15), preferably from about 1 to about 10microns, and most preferably from about 2 to about 5 microns.

The polymer coating is preferably applied to the substrate membrane overan area which is substantially co-extensive with the surface of saidmembrane.

The ability of the above described polymer coated microporous substratemembranes to achieve the aforedescribed properties renders thempaticularly suitable for use as battery separators in any primary orsecondary battery and particularly for any zinc or silver electrodesecondary battery. When employed for this purpose the number of coatinglayers and their arrangement on the microporous substrate membrane canvary and is governed by the needs of the battery in terms of itsperformance. Thus, using a microporous film as an example of a suitablesubstrate, the film may have the polymer coating on one or both sidesthereof. Alternatively, two microporous films having a single polymercoating on one side of each film may be arranged together with thecoated film surfaces facing each other or facing away from each other.This same type of arrangement can be achieved by folding a singlemicroporous film which is coated on only one side thereof so that thecoated surfaces of each half are faced as described above.

The polymer coated films because of their increased hydrophilicity canbe used to perform any function commonly performed by conventionalmembranes, preferably microporous membranes which are employed inbattery separators.

For example, the polymer coated membrane can be used to wrap varioustypes of films and sheets of dendrite barrier layers such as disclosedin U.S. Pat. Nos. 3,539,396; 3,970,472; and U.S. patent application Ser.No. 125,195, filed Feb. 27, 1980 entitled "Coated Open-celledMicroporous Membranes", by H. Taskier, the disclosures of which areherein incorporated by reference. These patents and application disclosebattery separators which contain nickel layers which must beelectrically insulated from the electrodes. Such insulation can beprovided by the polymer coated membranes of the subject invention.

Alternatively, the polymer coated membrane can be employed as a dendritebarrier layer itself by using one or more layers of polymer coatedmembrane to control dendrite growth.

The batteries in which the polymer coated membrane can be employed as aseparator include any primary or secondary cell which uses an aqueousalkaline electrolyte.

The preferred battery is a secondary battery which employs a zinc anodeand any conventional positive electrode suitable for use with alkalineelectrolytes including mercury oxide, manganese oxide, silver oxide, andpreferably nickel oxide.

Suitable alkaline electrolytes which are employed in conjunction withsuch electrodes include aqueous solutions of potassium hydroxide,lithium hydroxide, sodium hydroxide and mixtures thereof atconcentrations of from about 20 to about 45%, by weight, based on theweight of the solution.

The above described polymer coated microporous substrate membranes offera unique set of advantages which heretofore have been unachieveable inthe prior art. Such coated substrate membranes are extremely compact andmuch thinner than the metal coated screens, non-woven and woven cloths,nettings perforated plates and diffusion membranes of the prior art.Consequently, the power to weight ratio of batteries employing thesubject coated membranes can be increased substantially since many morecells can be constructed in a given area.

Moreover, because of the thinness of the coated membranes and their lowelectrical resistance, the use of the same as a battery separatorreduces the internal resistance of the cell compared to other knownthicker battery separators thereby increasing the cell's efficiency, andcapacity. In addition, the coated microporous substrate membranesdescribed herein are flexible, possess good mechanical properties, andare extremely cost efficient to manufacture, as well as extremelyefficient in their operation. They can withstand repeatedcharge-discharge cycling without gradual loss of power, and result in acell which is extremely resistant to shape change.

The following Examples are given as an illustration of the claimedinvention. It should be understood, however, that the invention is notlimited to the specific details set forth in the Examples. All parts andpercentages in the Examples as well as in the remainder of thespecification are by weight unless otherwise specified.

EXAMPLE 1 PART A

Crystalline polypropylene having a melt flow index of about 5.0 and adensity of 0.905 gm/cc is melt extruded at a temperature of about 230°C. through a 12 inch diameter blown film die. The film is inflated toprovide a blow-up ratio, or D₂ /D₁ ratio, of 1.1. The film is then takenup at a drawndown ratio of 75:1. The non-porous precursor film producedin this fashion is found to have the following properties: thickness,about 1 mil (0.001 inch); recovery from 50% elongation at 25° C., 90.5%;crystallinity, 68.8%.

A sample of this film is oven annealed with air with a slight tension at145° C. for about 23 minutes, removed from the oven and allowed to cool.

The sample of the annealed elastic precursor film is then subjected tocold stretching and hot stretching at an extension ratio of 0.8, i.e.,20% cold, 80% hot, and thereafter heat set under tension, i.e., atconstant length, at 145° C. for 10 minutes in air. The cold stretchingportion is conducted at 25° C., the hot stretching portion is conductedat 145° C., and total draw is 100%, based on the original length of theelastic film. The resulting film has an open-celled pore structure asdefined herein, an effective pore size of about 400 Angstroms, aporosity of 45%, a density of 0.49, a crystallinity of about 59.6%, anda thickness of 1 mil. Several film samples are prepared as describedabove and each is designated Film A.

PART B

Part A is repeated with the exception that the polypropylene employed toform the precursor has a melt flow index of about 0.7, and a density of0.920 gm/cc.

The resulting microporous film exhibits an effective pore size of about200 Angstroms, a porosity of 38%, a density of 0.56 gm/cc and athickness of 1 mil. Several samples are prepared and designated Film B.

PART C

Microporous film samples prepared in accordance with Part B are employedas a substrate for the deposition of a non-woven felt embossed layer.

Accordingly, polypropylene is spray spun at a melt temperature of 350°C. through a 0.016 inch diameter nozzle, using jets of steam superheatedto 405° C. and at 21 psi to attenuate the melt stream of polymer into acontinuous filamentary material characterized by a denier per filamentranging from about 1 to about 4. The filamentary material is collectedon a revolving drum having a smooth metal surface spaced from theextrusion orifice at a distance of about 2 feet. The drum is rotated ata speed and for a time sufficient to produce a non-woven web weighingabout 0.5 oz. per square yard. The resulting non-woven web is thenlaminated to samples of Film B using an embossing roll heated to atemperature of about 120° C.

The resulting felt coated embossed film has a thickness of about 4 mils.These film samples are designated Film C.

PART D

A microporous film sample prepared in accordance with Example 1, Part Ais coated on one side with a nickel coating by means of a sputteringtechnique. The thickness of this coating is about 350 Angstroms. Theresulting nickel coated film is referred to herein as film D.

PART E

The following components are mixed together as shown at Table 1.

                  TABLE 1                                                         ______________________________________                                        Component        Parts by Wt.                                                 ______________________________________                                        Cellulose acetate.sup.(1)                                                                      5                                                            Victawet 12.sup.(2)                                                                            5                                                            Acetone          90                                                                            100                                                          ______________________________________                                         .sup.(1) The cellulose acetate is characterized by an acetyl value of 55      ± 0.35, a viscosity at 25° C. as determined from a 6% solution      thereof in acetone of 105 ± 25 cps, and a density of about 23 gm/cc.       .sup.(2) Victawet 12®  is a phosphoric ester surfactant of ethoxylate     2ethyl hexyl phosphate available from Stauffer Chemical Co.              

The resulting composition is applied to both sides of a sample of Film B(Part B of Example 1) using a #16 coating bar. The resulting filmcoating has a thickness of 0.1 mil, the coated film has an averagecoating weight of 1.76 gm/ft² and the weight of the coating is 0.4gm/ft² of film surface or an add-on of 29.7%, by weight, based on theweight of the uncoated film. The resulting coated film is dried in anoven at 175° F. and then tested immediately for electrical resistanceusing a 40% solution of KOH as described herein which is found to be 6milliohms-in² (average). These results are also shown at Table II, run4.

PART F

Several additional polymer film coated samples are prepared inaccordance with Part E and the identity of the substrate membrane, thenumber of sides coated with the polymer, the composition of the polymersolution from which the polymer coating is applied, and surfactant arevaried. The appropriate processing conditions employed in the coatingprocedure are shown at Table II.

Three control samples (i.e., runs 1 to 3) are prepared by immersingmicroporous film samples in a solution of a surfactant for a period oftime sufficient to obtain an add-on of surfactant as shown at Table II.The silicone glycol-imidazoline surfactants employed comprise a 6%solution of a blend of an imidazoline surfactant available under thetrademark Wictamid AL42-12™ from Witco Chemical and a polyoxyethylenepolymethyl siloxane surfactant available under the tradename Dow-193™.The weight ratio of each surfactant in the blend is 1:1 and the solventis acetone. The add-on of surfactant is determined after the solvent hasevaporated therefrom. When Victawet-12 surfactant is employed it isapplied to the control film samples from a 6% solution thereof inacetone.

Table II also shows the method of application of the surfactant to thefilm. The concentration of the polymer and the weight ratio of thepolymer to surfactant in the solution is shown at Table II.

The substrate film type is identified by the letters A to D whichcorrespond to film samples A to D in Example 1.

The electrical resistance of each film sample is measured and theresults shown at Table II.

As may be seen from the data of Table II the cellulose acetate coatedfilms in most instances exhibit a reduction in the electrical resistancein relation to the corresponding uncoated, microporous films treatedonly with surfactant. The lower E.R. of the film sample of run 10 inrelation to its corresponding control (i.e., run 3) is believed to beattributable to the lower amount of surfactant possessed by the run 10sample with respect to the latter. At similar surfactant levels thecellulose acetate coated film would be expected to exhibit a lower E.R.in relation to the control sample. Similar considerations also applywith respect to runs 7-9.

                                      TABLE II                                    __________________________________________________________________________                                         Wt.                                                                           Ratio                                                                         of Sur-                                                                            No. of                                                                             Add-on                                           Method    Sol-     factant                                                                            Sides                                                                              of                                   Sub-        of Sur-   vent                                                                              Polymer                                                                            to   of Film                                                                            Polymer                                                                            Add-on                          strate      factant   For Conc in                                                                            Polymer                                                                            Coated                                                                             and Sur-                                                                           of    E.R.                      Film                                                                              Surfactant                                                                            Appli-                                                                             Polymer                                                                            Pol-                                                                              Solution                                                                           in Solu-                                                                           With factant                                                                            Surfactant                                                                          (milli-             Run No.                                                                             Type                                                                              Type    cation                                                                             Type ymer                                                                              (%)  tion Polymer                                                                            (%)  (%)   ohms-in.sup.2)      __________________________________________________________________________    1 (control)                                                                         B   Victawet-12                                                                           immer-                                                                             None N/A N/A  N/A  None N/A  14-20 6                                     sion of                                                                       uncoated                                                                      film in                                                                       6% sur-                                                                       factant                                                                       solution                                                                      of ace-                                                                       tone                                                        2 (control)                                                                         B   silicone glycol-                                                                      immer-                                                                             None N/A N/A  N/A  None N/A  12-17 8                             imidazoline                                                                           sion of                                                               blend   uncoated                                                                      film in                                                                       6% sur-                                                                       factant                                                                       solution                                                                      of ace-                                                                       tone                                                        3 (control)                                                                         A   Victawet-12                                                                           immer-                                                                             None N/A N/A  N/A  None N/A  14-20 2.5                                   sion of                                                                       uncoated                                                                      film in                                                                       6% sur-                                                                       factant                                                                       solution                                                                      of ace-                                                                       tone                                                        4     B   Victawet-12                                                                           PS   CA   ace-                                                                              5    1:1  2    29.7 14.9  6                                               tone                                              5     B   Victawet-12                                                                           PS   CA   ace-                                                                              5    1:1  1    20   10    5                                               tone                                              6     B   Victawet-12                                                                           PS   CA   ace-                                                                              5    1:1  2    30   15    4.6                                             tone                                              7     B   Victawet-12                                                                           immer-                                                                             PVA  eth-                                                                              10   N/A  1    15-20                                                                              7.5-10                                                                              8                                     sion of   anol                                                                uncoated                                                                      film in                                                                       6% sur-                                                                       factant                                                                       solution                                                                      of ace-                                                                       tone                                                        8     B   Victawet-12                                                                           immer-                                                                             PVA  eth-                                                                              5    N/A  2    20-30                                                                              10-15 9                                     sion of   anol                                                                uncoated                                                                      film in                                                                       6% sur-                                                                       factant                                                                       solution                                                                      of ace-                                                                       tone                                                        9     A   Victawet-12                                                                           immer-                                                                             PVA  eth-                                                                              5    N/A  2    20-30                                                                              10-15 5                                     sion of   anol                                                                uncoated                                                                      film in                                                                       6% sur-                                                                       factant                                                                       solution                                                                      of ace-                                                                       tone                                                        10    A   Victawet-12                                                                           roll coat-                                                                         CA   ace-                                                                              5    1:1  1    20   10    3                                     ing bar   tone                                                                using 6%                                                                      surfac-                                                                       tant solu-                                                                    tion in                                                                       acetone                                                     11    C   silicone                                                                              immer-                                                                             None N/A N/A  N/A  N/A  N/A  8     22.2                          glycol-imida-                                                                         sion in                                                               zoline blend                                                                          6% sur-                                                                       factant                                                                       solution                                                                      of ace-                                                                       tone                                                        12    C   *       **   CA   ace-                                                                              5    1:1  1    18   12    14.8                                            tone                                              13    B   Victawet-12                                                                           PS   CTA  meth-                                                                             2.5  2.1  1    40   20    4.2                                             ylene                                                                         chlor-                                                                        ide/                                                                          meth-                                                                         anol                                                                          (9:1)                                             14    D   Victawet-12                                                                           PS   CA   ace-                                                                              5    1:1  nickel                                                                             26   13    1.5                                             tone          side                                __________________________________________________________________________     N/A = not applicable                                                          CA = cellulose acetate (see description in Part D)                            PVA = polyvinylalcohol (number average mol. wt. 133,000; % hydrolyzed         99.8%)                                                                        CTA = cellulose triacetate  acetyl value 61.6 +  .25/- .04                    PS = polymer solution                                                         * = Uncoated film is first impregnated with the silicone glycolimidazolin     blend.                                                                        ** = The presurfactant impregnated film is then coated with a polymer         solution which contains a different surfactant, 1.e., Victawet12.        

EXAMPLE 2

The following example serves to illustrate the reduction in poreplugging achieved by use of the coated microporous film describedherein.

A microporous film sample prepared in accordance with Example 1, Part Eis provided, and a control film sample which is prepared by impregnatingFilm B (see Example 1, Part B) with Victawet 12 surfactant (i.e., byimmersing in a 6% solution of surfactant in acetone) to achieve anadd-on of surfactant of about 15%, based on the weight of themicroporous film alone. These two film samples are immersed in separatevessels containing a 31% aqueous solution of KOH saturated with zincoxide for a period of about 48 hours. Specimens from each film sampleare removed from the solution and blotted dry.

Both samples are then observed visually under a microscope. The controlfilm sample containing only surfactant exhibits a white opaqueappearance and when viewed under a microscope fine white particles ofzinc oxide are observed imbedded throughout the film. In contrast thepolymer coated film, when viewed under a microscope exhibits a finewhite deposit only on the surface of the polymer coating.

The air Gurley (ASTM D-726 B) of the polymer coated film sample afterimmersion is determined to be 68 seconds. This film sample is thenwashed with water and dried and the air Gurley again measured and foundto be 31 seconds. The polymer coated film sample is then soaked inacetone to remove the polymer coating and the air Gurley value is foundto be 32 seconds.

The dry weights of both film samples are measured before and afterimmersion in KOH-zinc oxide solution to determine the add-onattributable to zinc-oxide. The add-on of zinc oxide of the control filmsample is found to be 20%, by weight, and the add-on of the polymercoated film sample is found to be 5%, by weight, based on the weight ofthe respective film samples just prior to immersion.

The above test results indicate that penetration and absorption of themicroporous film by zinc ozide is substantially reduced by the polymercoating.

EXAMPLE 3

The following example is conducted to illustrate the improvement inhydrophilicity, by measuring the electrical resistance, and electrolyteabsorption of the polymer coated microporous films.

A film sample coated with a cellulose acetate-surfactant solutionprepared in accordance with Example 1, Part E, and a control filmsample, i.e., Film B (see Example 1, Part B) impregnated with a 6%solution of Victawet 12 surfactant in acetone to achieve a surfactantadd-on of about 15% are dried for 24 hours at 150° F. and allowed tocool to ambient conditions and weighed. Both film samples are immersedin a 40% aqueous solution of KOH for 24 hours. Specimens of each filmsample are cut into 2 inch squares and their weight recorded.

The electrical resistance (as described herein) and electrolyteabsorption are then determined and the results shown at Table III.

                  TABLE III                                                       ______________________________________                                                       E.R.        Electrotype                                        Film Sample Type                                                                             (milliohms-in.sup.2)                                                                      Absorption (%)                                     ______________________________________                                        Polymer coated                                                                film sample    7.1         106                                                control        8.5          68                                                ______________________________________                                    

The above data illustrates that both the electrical resistance andelectrolyte absorbtion of the cellulose acetate polymer coated film aresubstantially improved in relation to the control.

EXAMPLE 4

The following Example illustrates the dimensional stability of thepolymer coated films of the present invention in relation to cellophanefilms.

Cellulose acetate coated microporous films prepared in accordance withExample 1, Part E, are immersed in a 40% aqueous solution of KOH for 48hours. The length, width and thickness of the film sample is measuredbefore and after immersion, i.e., dry and then wet. The length ismeasured in the machine direction (i.e., direction of cold and hotstretch during film formation), the width is measured in the transversedirection (i.e., perpendicular to the direction of cold and hotstretching during film formation) and the thickness in mils. The percentchange of each dimension based on the initial film dimensions is thendetermined. The above procedure is then repeated using a cellophane filmavailable from E. I duPont de Nemours & Co., Inc. under the tradenamePUDO-134™. The results are summarized at Table IV.

                  TABLE IV                                                        ______________________________________                                                    Thickness  Width      Length                                      Film Sample (% change) (% change) (% change)                                  ______________________________________                                        Polymer coated                                                                microporous film                                                                          +9.1       +.75       -0.25                                       PUDO-134®                                                                             +192       +4.7       -2.0                                        ______________________________________                                         NOTE: A positive % change represents swelling and a negative % change         represents shrinkage.                                                    

The above data clearly shows that films of the subject invention exhibitsubstantially more dimensional stability than conventional cellophanefilms.

The principals, preferred embodiment and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A hydrophilic open-celled microporous membranewhich comprises a normally hydrophobic microporous membrane, havingdeposited on at least one surface thereof a coating comprising a polymerselected from the group consisting of cellulose esters, and polyvinylalcohol, said coating having a dry thickness of from about 1 to about 25microns, and a surfactant disposed within said coated microporousmembrane in a manner and in at least an amount sufficient to render thesubstrate microporous membrane hydrophilic.
 2. The coated microporousmembrane of claim 1 wherein the cellulose ester is cellulose acetate. 3.The coated microporous membrane of claim 1 wherein said coatinginitially additionally comprises a surfactant.
 4. The coated microporousmembrane of claim 1 wherein said substrate membrane has from about 8 toabout 40%, by weight, based on the weight of said substrate membrane ofsurfactant impregnated in the pores thereof.
 5. The coated microporousmembrane of claim 1 wherein said surfactant is initially present in saidcoating in an amount of from about 20 to about 200%, by weight, based onthe weight of said polymer coating.
 6. The coated microporous membraneof claim 1 wherein the surfactant is selected from the group consistingof silicon glycol copolymers; mixtures of silicon glycol copolymers andat least one imidazoline tertiary amine; ethoxylated 2-ethyl-hexylphosphate; vinyl acetate; acrylic acid; and methacrylic acid.
 7. Thecoated microporous membrane of claim 1 wherein the membrane is anopen-celled microporous film, derived from an olefinic polymer by the"dry stretch" or "solvent stretch" methods, having an average pore sizeof from about 200 to about 10,000 A, a porosity of from about 30 toabout 85%, and a surface area of at least 10 sq.m/gm; and said coatinghas a dry thickness of from about 1 to about 10 microns.
 8. The coatedmicroporous membrane of claim 1 wherein the membrane upon which saidcoating is deposited further comprises a layer of non-wovensubstantially continuous randomly arranged filamentary material derivedfrom polymers selected from the group consisting of polyolefins,cellulose acetate, polyamides, polyacetals, polyalkylene sulfides, andpolyarylene oxides, said layer of filamentary material being embossed onthe surface of said membrane to achieve a thickness of from about 1 toabout 10 mils, and being coextensive with the surface of the microporousmembrane on which it is present.
 9. The coated microporous membrane ofclaim 8 wherein the filamentary material is derived from a polyolefin,said filamentary layer is applied to only one surface of the microporousmembrane and said coating is present on the surface of said filamentarylayer.
 10. The coated microporous membrane of claim 8 wherein thefilamentary material is polypropylene having a denier per filament offrom about 0.5 to about 5 and the dry thickness of the filamentary layeris about 1 to about 5 mils.
 11. A battery separator which comprises atleast one of the coated microporous membranes of any one of claims 1through
 10. 12. In a battery comprising at least one electrolytic cell,said cell including an anode and a cathode disposed in an alkalineelectrolyte, said anode and cathode being separated in said electrolyteby a battery separator the improvement comprising the battery separatorcomprising at least one coated microporous membrane of any one of claims1 to
 10. 13. The battery of claim 12 which is rechargeable and containsa zinc anode.